Bachelor cum laude, Carleton College, Northfield, Minnesota, 1989. MSJ with honors, Medill School Journalism, Northwestern University, 1993.
Monika Bauerlein is the other co-editor. She got a Master's degree from the Medill School of Journalism at Northwestern University in 1993. Between 1993 and 1995, Jeffery was a staff editor and writer at Washington City Paper.
She was a senior editor at Harper's magazine (1995–2002), where she edited six articles nominated for a National Magazine Award, including essays by Barbara Ehrenreich that became Nickel and Dimed, and several anthologized in the "Best American" series. She became deputy editor of Mother Jones, a position she had held for four years, and was promoted to co-editor in August 2006,
Together, Jeffery and Bauerlein have aimed to put greater emphasis on staff-generated, daily news and original reporting. The magazine received a National Magazine Award for General Excellence in 2008 and 2010.
In 2012 Mother Jones broke the story about Mitt Romney's "47 percent" remarks, which were controversial prior to Barack Obama winning reelection. In 2002, Jeffery wrote an article on the Salton Sea for Harper's, "Go West Old Man: Where the American Dream Goes Down the Drain", which received an honorable mention in Best American Science and Nature Writing. She has also written for Slate, the Huffington Post, San Francisco Magazine, and the Chicago Reporter.